To play the game, the players must compete with each other to build a complete city with 8 districts (though the number can be reduced to shorten the game's play time). The game ends when a player completed their city, and the player with the highest amount of points wins.
The players begin each round by selecting 1 of 8 character cards, each possessing a particular skillset:
- Assassin: Kills another character, and the murdered character would not have a turn in that round.
- Thief: Steals all the gold from another character except the assassin and the assassin's target.
- Magician: Can trade hands with another player, or discard any number of cards from hand and taking a new selection from the deck.
- King: Receives the "king" token, which means they would initiate the character selection and choose the first character card. Once per turn, they can gain an extra gold for every yellow-coloured district they have at that moment.
- Bishop: Once per turn, they can gain an extra gold for every blue-coloured district they have at that moment. Also, they're immune to the skills of rank 8 characters.
- Merchant: Receives 1 extra gold per turn. Also, once per turn, they can gain an extra gold for every green -coloured district they have at that moment.
- Architect: Can draw 2 extra cards. They can also build up to 3 districts at once.
- Warlord: Can destroy another player's district (by paying the cost of the particular district minus 1 gold). Once per turn, they can gain an extra gold for every red-coloured district they have at that moment.
At the start of the game, each players are given 4 district cards, and they play their turns by the order of the numbers of their character cards. During their turn, the players can take the following actions: draw 2 more district cards or collect two gold pieces, use their characters' skills and, if they have enough gold pieces, build a district (unless they were killed by the Assassins, during which they would not be able to do any of these). After each round, the characters are re-shuffled, and the cycle begins anew.
Each district cards have a specific point value, based on their building cost (i.e. a district that costs 5 gold to build would be worth 5 points, though the point would only be counted if you actually build them). Each cards are also given different colour codes (red, yellow, green, blue and purple), which can give the players coin benefits depending on the character cards they have. The purple cards are "special districts" which have their own unique powers. Accumulated gold will not be counted to the players' final score.
Three expansion packs have been released: the "Additional character set" edition, which provides a set new characters that can be swapped in with the old cast based on their numerical counterparts, the "Dark City" edition, which adds 14 new purple district cards, and the "Circus" edition, which features new "Action cards" that offers new tactical advantages.
A new edition of the game was released in 2016 which included most of the content from the original game and the Dark City expansion pack as well as some brand new content plus improved artwork. Yet another new, more compact edition of the 2016 version was released in 2021 simply called "Citadels Revised Edition" which also came with its own limited mini-expansion called "Citadels: Scene Route" which added some more new district cards to the game.
Citadels is also now available to play online with some slightly altered rules.
- Witch: Bewitches another character; a bewitched character cannot use skills nor build districts, while the Witch can build districts and use the skills of the bewitched character. However, if the Witch attempts to bewitch a character that isn't held by any player, she loses the chance to build a district for that turn.
- Tax Collector: If the Tax Collector is one of the characters used in the game, every player must pay a tax whenever they build a district, paying 1 gold into a separate reserve. A player using the Tax Collector character doesn't pay the tax, and instead can take the reserve and add it to their own stash.
- Wizard: Can see another player's hand and steal one card; the stolen district can be immediately built without counting for the "one district built per turn" limit, or added to the Wizard's hand. Also, the Wizard can build a district even if there's already a district with that name in his city.
- Emperor: Must take the "king" token from the current player and give it to someone else (not himself); in exchange, he takes one coin or card from the crowned player. Once per turn, they can gain an extra gold for every yellow-coloured district they have at that moment.
- Abbott: Once per turn, they can gain an extra gold and/or card for every blue-coloured district they have at that moment. Also, once per turn, they can take one gold from the richest player - but only if they themselves aren't the richest player.
- Alchemist: At the end of their turn, regains all gold spent on building districts.
- Navigator: They can either gain 4 extra gold, or draw 4 extra cards. However, they cannot build districts.
- Diplomat: Can trade one of their own districts in exchange another player's district (paying the gold difference if the other player's district is more expensive). Once per turn, they can gain an extra gold for every red-coloured district he has at that moment.
Additionally, the expansion has two characters with the number 9:
- Artist: Can beautify up to two districts per turn, spending 1 gold each. A beautified district has a cost 1 gold higher than normal.
- Queen: Gains 3 gold if they're seated next to a rank 4 character (King or Emperor), even if that character was assassinated.
In the new 2016 edition, another 9 characters were added:
- Magistrate: Can send three face-down warrants towards three other characters. One warrant is signed; when that character builds their first district from that turn, the Magistrate can reveal the signed warrant and add the district into his own city, while the targeted player regains the gold spent.
- Blackmailer: Can send two face-down threats (one marked) towards two other characters. A threatened player can decide to either pay half of their gold, or call the Blackmailer's bluff; in the latter case, the Blackmailer can flip the token and - if it's marked - take all the target's gold.
- Seer: Can take one random card from every other player, then give one card of their choice from their own hand to each player. They can also build two districts at once.
- Patrician: Works similarly to the King, except they get extra cards instead of extra gold.
- Cardinal: Once per turn, they can gain an extra gold for every blue-coloured district they have at that moment. Also, if they don't have enough gold to build a district, they can take the necessary gold from one other player, giving back an equal amount of cards in return.
- Trader: Once per turn, they can gain an extra gold for every green-coloured district they have at that moment. For that turn only, green districts do NOT count for the "only one district built per turn" rule.
- Scholar: Can draw seven cards, add one to their hand, and discard the rest. They can also build two districts at once.
- Marshal: They can take another player's district by paying its cost in gold, but only if its cost is 3 gold or less. Once per turn, they can gain an extra gold for every red-coloured district he has at that moment.
- Spy: Can name a district color and a player. For every card of that type in that player's hand, the Spy steals one gold from them, and draws one district card from the deck. It's actually another #2 card, relegating the Tax Collector from his previous position to a #9 card.
Citadels provides examples of the following tropes:
- Anti-Hoarding: Players are free to stockpile resources and there are even a few effects that give end of game bonuses for holding onto cards and coins. That being said, hanging onto vast amounts of cards and money is extremely risky due to a number of special abilities that allow other players to take or otherwise remove them. A bulging wallet does a player no good when it's all stolen by a rival...
- Colour-Coded for Your Convenience: There are 5 different types of districts, each coded with a different colour and provides various advantages for the players. Having all 5 district types in your city will grant you 3 extra points at the end of the game.
- Royal/Noble districts (Yellow): Gives 1 extra gold for the King, the Emperor, or the Patrician.
- Religious districts (Blue): Gives 1 extra gold for the Bishop, the Cardinal, or the Abbot.
- Trading districts (Green): Gives 1 extra gold for the Merchant or the Trader.
- Military districts (Red): Gives 1 extra gold for the Warlord, the Diplomat, or the Marshal.
- Special districts (Purple): Various. Special benefits is inscribed in the individual district cards.
- Construction and Management Games: The main goal of the game is to expand your territory by building as many high-value buildings as possible.
- Crippling Overspecialization: The Navigator can get tons of resources in a single turn, but they cannot build any district.
- Death Is Cheap: Since the characters are re-shuffled every round, anyone who has been killed by the Assassin will be "revived" in the next round.
- Difficult, but Awesome:
- The Witch has one of the most powerful effects in the game, allowing her to steal an opponent's character ability and preventing them from building districts on top of that. As a tradeoff, if she accidentally targets a character that is not being used by any player, she doesn't gain any ability, and instead loses the chance to build districts for that turn.
- The University and the Dragon Gate are the most expensive districts, costing 6 gold each. Due to their awesomeness, they're worth 8 points.
- Foil: Characters with the same rank tend to be foils of each other and share common themes:
- 1: Characters that can disable the actions of other characters.
- 2: Furtive characters. They steal gold from other characters.
- 3: Magic users. Their skills manipulate players' hands (their own or someone else's).
- 4: Royalty. They gain resources from yellow districts, and can manipulate the "king" token.
- 5: Religious figures. They gain resources from blue districts, and their skills force other players to "help".
- 6: Characters involved with money. Their skills revolve around gaining gold and/or green districts.
- 7: Scholars and experts. Their skills revolve around drawing cards and/or building districts.
- 8: Military figures. They affects districts built by other players, and gain resources from red districts.
- 9: Other characters that don't fit the previous categories.
- For Doom the Bell Tolls: The "Bell Tower" unique district card from the Dark City expansion is an ominous structure that allows the player to change the build limit from eight districts to seven and essentially cut the game short by a turn. Great for suddenly ending the game right before everyone can get their final builds out and stealing a last minute victory.
- Lighthouse Point: The Dark City district card "Lighthouse" lets the player who built it search the district deck and take any card that want. Quite powerful for something that only costs 3 coins to make.
- Low Fantasy: The selectable characters includes a Magician (and, from the expansion packs, a Witch, a Wizard, and a Seer), while one of the districts is called the School of Magic. Despite this, magic doesn't play a significant role in the gameplay (since the main focus of the game is to build cities and not RPG-based combat), and no non-human characters appear to exist in the game's setting.
- Manipulating the Opponent's Deck: All the no. 3 cards. The first edition has the Magician, who can force to trade a card trade with any other player (alternatively, it allows you to discard any number of cards from your own hand and replace them with new cards from the main deck). The additional set gives the Wizard, who can see another player's hand and steal a card. The 2016 adds the Seer, who can take one random card from every other player, then give one card of their choice from their own hand to each player.
- Medieval European Fantasy: The designs of the characters and districts combines Renaissance art-style and Standard Fantasy Setting costumes and locations.
- Non-Lethal K.O.: Having a "Hospital" unique district from the Dark City expansion downgrades the Assassin into this, but only for the player who actually owns it. While the attacked player still can't build any districts for that turn they can at least collect their resources, implying a nonfatal wound and some bedrest for a while. Better luck next turn.
- No Plot? No Problem!: Although the game provides a very elaborate setting with rather complicated rules about the characters' dynamics and district-based strategy, there's no actual plot/storyline beyond "build better districts than the other players to earn the most points by the end of the game". The characters don't even come with any backstory, since they are not meant to be individual persons, but a representative of various classes/occupations within a kingdom.
- No-Sell: The Assassin, the Witch and the Magistrate are all immune to the Thief.
- The Bishop is immune to the Warlord, the Diplomat and the Marshal.
- Place of Protection:
- An interesting variation, as the protection comes from the character, rather than the actual place they are affiliated to, but if you are playing as the Bishop, your districts cannot be touched by the Warlord, Diplomat or Marshal.
- The "Keep" district is this. As a result, it cannot be affected by the Warlord.
- Puppet King: The Emperor, unlike the King and the Patrician, doesn't take the crown token; instead, they give it to someone who isn't the Emperor itself or the previous owner. Then they get paid a tribute by the newly-crowned player.
- Pyrrhic Victory: The winner is not the player who first "completed" their own city (by building 8 districts), but who has the most points. Because of this, a player can fill their own city with cheap districts and end the game, only to discover that their opponent - who has less districts, but richer and more variegated - has more points. It's downplayed because players receive bonus points if they complete their city (and even more if they were the first in doing so).
- Purple Is Powerful: The Special district cards, which gives some of the best advantages in the gameplay, are coded with purple.
- Scenery Porn: All of the district cards have some breathtakingly fantastic artwork, especially the Unique districts which typically feature much more detailed and diverse designs.
- Treasure Room: The "Imperial Treasury" district gives players a point bonus for holding onto gold at the end of the game.
- Wishing Well: The "Wishing Well" is a Unique district that sadly doesn't have any effects related to gold or wishes and just provides a small amount of bonus points depending on the number of Unique districts a player has built in their city.